I'll be first to say Diamond Sea by Sonic Youth then...! It's obvious but still very special.
To add to that Goldie's Mother from the two disc set of Saturnz Return was quite a feat of work that gets a spin every so often, that whole album's a bit under-rated though.
My bandmate told me to listen to Sleep's Dopesmoker a while ago, I only have 20 min live version but quite enjoyed that, the recorded version is about 40...?
Smashing Pumpkins bootlegs from 1996 have some epic stretched out version of Silverfuck that are well worth a listen, so of them run out of steam or meander off but a few of them are amazing jams, I also love the 15 min+ versions of Joy Division's Transmission they were playing on the Adore tour.

Fantomas released a 76 minute album didn't they? Has anyone listened to the Flaming Lips' 24 hour song?

Well, isn't pretty much every song in the world ever, around the 3 to 6 minute mark? With a few exceptions of course - too small, percentage wise for me to reign my neck back in in respect to the tone of this post.

With that pretty well known fact, shall we set the parameters at around 9 minutes and over - just to be on the safe side?

awe-inspiring. I've spent a fair few hours suspended adrift on its radiant shimmer.

Bardo Pond - Amen (29:15)

oh man. that heavy, droning church-bell sound is monumental. and the thick miasma of psych drone guitar beats anything Dylan Carlson laid down in the same era.

Godspeed You Black Emperor! - Providence (29:02)

my favourite Godspeed moments are the drones. this piece has my favourite: the drone that seeps in at around 17/18 minutes. the unearthly rotary modulation or whatever it is...phasing, flanging, who knows what they did to make that guitar so ancient and lambent. they way it moves ever so subtly into a sad major and then back to minor kills me.

Stars of the Lid - The Artificial Pine Arch Song (18:01)

this is the sound of the subtle shifting gradients of post-sunset light, imperceptible in the way its weightless glow changes hue.

Do Make Say Think - Goodbye Enemy Airship (12:37)

tension...release. this is the finest execution of said technique I've ever heard. masterful. the restraint is admirable, in the build-up and the dénouement.

(and not counting symphonies /other classical works with multiple movements, even though you'd have to be a monster to not listen to them all the way through anyway), here's six of my absolute favourites:
The KLF - Chill Out (I own a gapless version of it, therefore it is one long piece. And besides, nobody would ever listen to just one bit of it anyway.)
Hugo Alfvén - Symphony No. 4
Kashiwa Daisuke - April.#02
Fela Kuti - Confusion, pts.1&2
Kraftwerk - Autobahn
Tortoise - Djed

All six would be straight into a "favourite pieces of music" list regardless of any demands about long pieces. In the case of all except maybe Fela and Kraftwerk, they're simply the best things those artists have done.

Would also include William Basinski's Disintegration Loop I in there but it is pretty long. And would have liked to include December Hunting for Vegetarian Fuckface - SOTL and Everlasting Light by Mono.

1)Funkadelic- Maggot Brain
http://www.youtube.com/watch?v=dh3bleXWaCkProbably one of my favourite tracks ever. The guitar is just hypnotic. I tried to learn the solo once when I'd only been playing the guitar for about a year. Got about 2 bars in and it had taken me about 3 hours. Gave up- it was for the best.

Hallogallo by Neu!
Christmas Steps by Mogwai
My Father My King by Mogwai
Calvary Scars II by Deerhunter
Lush Life by John Coltrane
December Hunting For Vegetarian Fuckface by Stars Of The Lid
In The Air I-III by Tim Hecker
Plague Of Angels by Earth
...Antennas To Heavesn by Godspeed You! Black Emperor
The Sound by Swans
Jane Doe by Converge
Primitive Associations/Great Mass Above by Growing
Transmissionary by Lightning Bolt
Bhima's Theme by Om
All Blues by Miles Davis
Sunshine (no sun) Into The Sun by Harvey Milk

I am listening to the title track of Ellen Fullman's Through Glass Panes. It's pretty fantastic, she plays her "big stringed instrument" or whatever exactly she calls it, droning away, while somebody else bashes away at a fiddle or something similar. Great stuff, about 20 minutes long.